Measuring liquids with trapped air

Ron Davis

We are finding that air bubbles are getting trapped in the liquid during transfer operations. The trapped air is inflating our measurements, both by flow meter and by sensing tank level. The bubbles eventually work their way out of the liquid after a couple of hours.

Anthony Kerstens

First of all, how is the air getting in. Is it be excessive agitation, or by not waiting until the agitator props are adequately covered? Or is the air getting in through a sucking hole in piping? What about the pumps? Centrifugal or fixed displacement? The centrifugal pump can make froth of air bubbles.

Given the viscosity, your only hope is to eliminate air by prevention. I've seen filters tried on resins, but that only broke the air bubbles down to finer sizes. A settling tank might be used, but given the rise time for bubbles in resin, the tank might need to be huge.

RSewell

Air is comparatively high by volume but low in mass, therefore volumetric measurements will be much more distorted than mass measurements. If the desired, degassed product is pretty constant in density, the actual product volume can be inferred by a level measurement corrected for density. That is, liquid head is sensed near the bottom of the vessel, vapour space pressure is sensed at the top of the vessel, and density compensation is taken at some point between the bottom transmitter and below the expected operating level.

As for flow, a mass meter (coriolis) may be indicated for smaller lines but the application may be tricky and noisy if plugging happens.

DP flow devices yield mass flow but you will have to do some digging on mixed phase equations, check out Richard W. Miller: Flow Measurement Engineering Handbook for multiphase flow.

Bela Liptak

Having spent some 25 years in the plastics/polymer industry, I know how common is the swell/shrink effect due to the entrapment and release of air. I also know that swell varies with pressure, temperature, viscosity and only Murphy knows what else.

In case of level we normally would detect both the top level by noncontacting means (ultrasonic, microwave, gamma radiation) and the mass in the tank separately (by load cells).

In case of flow, we would either use direct mass flow sensors (if the process fluid will pass through a Coriolis w/o plugging) or would measure the mass flow indirectly by taking the product of the outputs of a volumetric flowmeter and of a density sensor.

jmGiraud

Not knowing more about the viscsity of the fuid, so that turbulent flow is attained, I would not recommend immediatly the orifice plate. But remember that it is a mass flow meter in its principle. So, for few bubbles of air, those have virtualy no influence. It only affects if your flow equation has been canibalised for volume. The vent hole in the plate is there to let trapped air or gas escape. [email protected]

Jon Watson

Multiphase fluid measurement is of critical importance to the oil industry. ISA (access via www.solartron.com) uses a helix volumetric meter with a density measurement to determine the individual phases and hence compensate the meter to produce the net mass flow of the component o interest. This might be done alternatively with any suitable volumetric meter and a suitable density measurement. Again, Solartron have a tube density meter with EGA amplifier which can measure the density of severely aerated fluids. To be effective pressure compensation may be required or knowledge of the density of the unaerated resin. In tank applications this may mean a pumped recirculation loop. They can also help with viscosity measurement (to get to the molecular weight in prepolymerisation control and have success with PIB, methyl methacrylate etc. but air is also a problem with viscosity measurement) Incidently, entrained air may have a serious effect on viscosity hence viscosity sensitive or pressure drop flow devices may not be reliable.